1. Field of the Invention
The present invention relates to a dielectric filter, and more specifically relates to a dielectric filter utilizing TM multiple-mode dielectric resonators for use in an antenna duplexer for example.
2. Description of the Related Art
A TM multiple-mode dielectric resonator, which is configured with a dielectric rod complex disposed within an outer conductive member and made up of a plurality of intersected dielectric rods, has been used as a bandpass filter. By using a TM multiple-mode dielectric resonator, a compact and high order dielectric resonator can be easily realized. In designing a dielectric filter, to attenuate unnecessary signals at the lower-frequency side or the higher-frequency side of the transmission band, an attenuation maximum is provided at the lower-frequency side or the higher-frequency side of the transmission band.
The inventors have already submitted Japanese Patent Application No. 6-160271. In that application, the technology is applied to a dielectric filter using a TM multiple-mode dielectric resonator. FIG. 21 is a view showing a configuration of an embodiment according to the invention disclosed in that application. In FIG. 21, there are shown TM double-mode dielectric resonators 10a and 10b. Dielectric rods 1a and 1b are provided with coupling loops 11a and 11b magnetically coupled therewith, respectively, and coupling loops 12a and 12b magnetically coupled therewith, respectively. Between the two dielectric resonators, a partition plate 14 is disposed in order to magnetically couple dielectric rods 2a and 2b and to prevent coupling between dielectric rods 1a and 1b. The coupling loops 12a and 12b are connected with a cable 13.
FIG. 22 is an equivalent circuit diagram of the dielectric filter shown in FIG. 21. This filter is a bandpass filter made up of four resonators in which the first resonator and the last resonator are coupled.
FIG. 23 shows the characteristics of the filter. When the first resonator is not coupled with the last resonator, the filter has the bandpass characteristics shown by curve B. With the first and the last resonators coupled, attenuation maximums are generated at the lower-frequency side and the higher-frequency side of the transmission band as shown by curve A.
In a conventional dielectric filter in which two coupling loops are connected with a cable in order to couple the first resonator with the last resonator, the number of components increases and the size of the filter also increases to provide room for the connecting cable. The cost of assembly rises, and adjustment becomes complicated. Adjusting the frequency of one attenuation maximum is not possible, since the two attenuation maximums generated respectively at the lower-frequency and higher-frequency sides of the transmission band move together. In other words, it is relatively difficult to independently adjust the respective frequencies of the attenuation maximums.